As is known, various techniques have been developed for powder metal to obtain products which cannot be economically fabricated from metal by employing other techniques, such as, casting, forging, stamping and the like. In a general sense, a mass of powder metal may be compacted into a three dimensional shape and thereafter sintered in order to obtain a bonding together of the various particles of the powder mass to form the final product.
U.S. Pat. No. 5,460,641 describes a metallic powder comprised of an assembly of granules, each of which is comprised of a group of spherically shaped elementary metal particles agglomerated by gelatin. Such agglomerated particles have been employed for making various products and, in particular, in making stainless steel products.
There are two prevalent methods of compacting agglomerated powder metal parts. The first is uniaxial pressing (movement of a pressing tool in a single direction), for example, as described in U.S. Pat. No. 5,460,641, in which a controlled amount of powder is automatically gravity fed into a precision die and compacted, usually at room temperature, at a pressure as low as 10 tons per sq. in or as high as 60 or more tons per sq. in (138 to 827 MPa) depending on the density requirements of the part (Metal Powder Industries Federation xe2x80x9cDesign Solutionsxe2x80x9d brochure).
It has been found that agglomerated powders as described in U.S. Pat. No. 5,460,641 cause binding of the punch and die, squealing and scoring of the tools, increased ejection pressure, tool breakage and production of parts which are outside acceptable deviation limits on weight and size.
Another commonly used pressing method, particularly for Scanpac powders, is cold isostatic pressing (CIP). Cold or room temperature compaction is carried out in liquid systems at pressures commonly reaching 60,000 psi (414 MPa). The metal powder can be packed into complex-shaped rubber or elastomeric molds before compacting. Free of die frictional forces, the powder compact reaches a more uniform density.
Powders with spherical or rounded particles are not cold compacted because of their inability to form a sound or strong green body.
Sintering can be performed by any of the conventional processes (Metal Powder Industries Federation xe2x80x9cDesign Solutions: brochure). The agglomerated Scanpac powders have sufficient green strength to form a cohesive component for sintering. Since no metal punches or dies are used in cold isostatic pressing, scoring or breakage of the tooling is not an issue but uniformity of product is an issue.
Accordingly, it is an object of the invention to provide an economical method for compacting agglomerate powder materials.
It is another object of the invention to provide a method for producing commercially useful powders for volume production.
It is another object of the invention to provide an agglomerated powder that can be compacted in a uniaxial manner without damaging the elements used to compact the powder.
Briefly, the invention provides a method of compacting agglomerated powdered metal comprised of the steps of receiving a mass of agglomerated powder of random sizes; separating the mass into at least three distinct range of sizes including a fine range, a medium range and a coarse range; and thereafter compacting the agglomerated powder of one of the ranges into a green briquette characterized in having a self-supporting structure to permit handling for a subsequent sintering operation to form a bonded compacted mass. The agglomerated powder of each of the other ranges may be separately compacted into a green briquette characterized in having a self-supporting structure to permit handling for a subsequent sintering operation to form a bonded compacted mass.
One advantage of the invention is that the shape and size for the bonded compacted mass can be predetermined and the agglomerated powder can be compacted to this shape and size without distortion of the bonded compacted mass, i.e. without a linear deviation from the predetermined size and shape.
Another advantage of the invention is that the stresses placed on the tools used for compacting can be uniform so that the tools do not skew during compaction.
Binder die wall lubrication is generally adequate when tools are above 150xc2x0 F. Above 150xc2x0 F., the binder material system gives higher green strengths at room temperature and adequate die wall lubrication. It is recommended that 0.5% of EBS (Ethylene Biostearamidexe2x80x94Acrawax xe2x80x9cCxe2x80x9d) be added to the powder to alleviate the lower temperature compaction lubrication inadequacy. This material delubes cleanly and is compactible with the binder. Addition of Acrawax xe2x80x9cCxe2x80x9d should be blended for a short blend time of 2 to 3 minutes in a non aggressive blender. Die wall lubrication has been found adequate and desirable with or without the Acrawax additions. The die wall lubricant should be a blend of 75% Acrawax xe2x80x9cCxe2x80x9d and Lauric Acid below 200 mesh.
Delubication is most critical before sintering. A gradual heating rate of briquettes to 888xc2x0 F. (475xc2x0 C.) is essential to remove the lubricant. The rate of heating in air should not exceed 20xc2x0 F./min. to reduce internal pressure build up or surface eruptions. Parts should be delubed on a carrier that will be used in sintering. Preference for the carrier would be a smooth ceramic plate compatible with the base material of the briquette and 2600xc2x0 F. temperature in 100% hydrogen.
Alternatively, the carrier should be made of a soft porous plate or surface that allows the lubricant and/or binder in the briquette to diffuse from the base of the briquette and that allows the briquette to slide on the carrier as the briquette shrinks during sintering. The porous plate or surface also allows the hydrocarbons in the briquette to escape rather than becoming entrapped and causing problems in the final product.
Sintering requires heating to 2540xc2x0 F. in 100% H2 or Ar partial pressure. Time at temperature should be 60 minutes.
Net Result is 99.73% of Theoretical Density.
In accordance with this invention, the segmentation and use of segregated mesh size particles and the addition of a selected lubricant eliminate the problems encountered with the previously known methods of compacting agglomerated powders and creates commercially useful powders for volume production.